1. Field of the Invention
The invention relates to a method of reading text printed by a matrix printhead, in which an area of the printed line is electro-optically scanned longitudinally and transversely at respective frequencies which are logically related to one another. A digital signal is generated thereby which reproduces the signal which controlled the printing. The invention also concerns a readhead device for implementing this method.
2. Description of the Prior Art
The most common electromechanical form of matrix printhead is the needle printhead, the operation of which is typical of matrix printing processes.
Needle printheads are currently the fastest electromechanical printing device. They comprise a plurality of adjacent cylindrical needles with their axes aligned transversely to the printing line, the head moving at substantially uniform speed along the line. Printing is produced by advancing certain needles, which press a transfer ribbon against a support sheet. At any time at which needles are advanced, to each needle there corresponds a point location, the combination of which constitutes a character body. Advancing a number of needles causes the printing of the corresponding points, the disposition of which defines the structure of the body. Needles are advanced to print a body in response to a command signal segment comprising a plurality of binary intervals in a sequence which corresponds to the structure of the body. Bodies succeed one another at an axial separation set equal to the location diameter. In alphanumeric printing, each character is formed by a number of adjacent bodies, often a plurality of bodies, the first and last of which are empty to constitute the gaps between characters.
Other types of printing are also possible, however. In particular, there is encoded printing of the kind disclosed in French patent application No. 79 16525 of June 27, 1979, in which each body represents a character in the ASCII code, with additional timing and parity data. This encoded printing in the margin of alphanumeric printing duplicates the clear language text in concentrated form and provides for reading this text in a directly usable form.
Nevertheless, it will be remembered that alphanumeric printing is controlled by a coded signal comprising multiple interval segments, using transcoding matrices such that to a signal segment there corresponds the number of bodies of which the alphanumeric character is composed. Thus the reading of alphanumeric text does not differ significantly from that of encoded text, since body by body scanning of the area occupied by a character produces a signal in one to one relationship with the printing control signal.
The immediately obvious method for reading a text consisting of a succession of character bodies, each body consisting of a plurality of circular point locations with the disposition of printed points corresponding to a determined structure, would seem to consist in consecutively scanning the locations of a body, to identify which locations are occupied by a printed point, and the bodies of the line, and determining the different reflection conditions from printed and empty locations with the scanning resolution being the same as the printing resolution.
While this reading method seems initially attractive by virtue of its simplicity, it is seen to be unusable in that it presupposes that the scanned locations are in strict positional conformity with the corresponding locations during printing. In other words, to use the terminology of printing, it assumes that perfect registration is obtained along and transversely to the line. This can only be achieved if coincidence errors between the printing format and reading format are less than a relatively small fraction of the location diameter at all points on the support. By way of indication, the needle diameter is often of the order of half a millimeter, and the maximum tolerable registration error is 20% of a needle diameter, that is to say 0.1 mm. Since in the A4 format the printing area normally measures 257.times.170 mm, or approximately 500 locations vertically and 350 locations horizontally, a systematic registration error of 0.2 micrometers per location is sufficient to produce the maximum permissible error at the end of the page, or a random error of 6 micrometers, assuming proportionality of the total error to the square root of the number of events subject to the random error.
Also, there are other sources of error, including the presence of dark spots on the printing medium in a blank area and failure to print part of a printed point. These printing errors result in reduced contrast when the location as a whole in analysed. Also, in the overall analysis of the point, determining the reflectivity produces an analog indication subject to the noise produced by each source of error, with the result that cumulative errors compromise the reliability of the classification between "1" and "0" logic states.
The objective of the invention is to provide a method of reading matrix printed text which accomodates and compensates for registration errors.
Another object of the invention is to provide a reading method which takes separate account of the various printing errors so as to compensate for them separately.